This invention relates to semiconductor integrated active filter circuits and more particularly, to such a circuit including distributed capacitor-resistor-capacitor (CRC) elements fabricated using MOS technology.
Resistors and capacitors as integral, but discrete, elements in semiconductor integrated circuits are well known. Resistors are fabricated within the bulk semiconductor material, on polysilicon material deposited on the surface of the device, or as thin film elements on both semiconductor and other substrates. Capacitors may be of the well-known MOS configuration or may utilize the multilevel conducting and insulating layers provided for semiconductor device interconnection on the surface of the device. The use of PN junction capacitance for various applications also is known. U.S. Pat. No. 4,092,619 discloses an MOS field device for use as a voltage controlled low-pass filter. However, integrated circuit resistors and capacitors often lack the linearity required for active filter performance. Or, in order to achieve the required linearity, the elements must be of a size which uses an inordinate amount of space in the semiconductor device. The dimensions of such elements might even preclude their integration. There are further difficulties, using MOS technology, in fabricating capacitors and resistors with adequate control of the performance characteristics of these elements.
It is important also in meeting the foregoing problems that the method of fabricating such passive elements departs as little as possible from the standard processing of the technology used to make the semiconductor integrated circuits.
Thus, there is a need for integrable capacitive and resistive components which are compact and are easily fabricated to precise parameters. Such elements advantageously should exhibit linear characteristics over their useful range. Such elements should be susceptible of facile integration into filter configurations for a variety of applications.